This dataset includes stream and rainfall hydrochemistry of the Plynlimon research catchments in Mid Wales. The data cover the period from March 2016 to March 2019. Sampling was carried out fortnightly from March 2016 to July 2017. From August 2017 to November 2018 stream samples were collected every four weeks and the rain sample every two weeks. From December 2018 onwards all samples were collected every four weeks. Data are presented for major anions and cations, pH, conductivity, alkalinity, in-situ measurements of water temperature and stream flow for the six stream locations, and air temperature and volume for the rainfall sites. Stream samples were taken using a grab technique and filtered in the field. Rain samples were collected using bulk precipitation collectors. Fieldwork was carried out by CEH members of staff trained in the fieldwork techniques required. Chemical analysis was carried out by qualified CEH chemists at laboratories at CEH Lancaster and CEH Bangor. The Plynlimon research catchments lie within the headwaters of the River Severn and the River Wye in the uplands of mid-Wales. Intensive and long-term monitoring within the catchments underpins a wealth of hydrological and hydro-chemical research. Monitoring is funded by the Centre for Ecology & Hydrology, and is ongoing since 1968.